Where to find "dragon tendon" when crafting steel cavalry equipment?

Dragon tendon is the rare item for steel cavalry parts. You can make then in the blacksmith building, tap the building and then the materials option. then select the steel cavalry option and add the tendon parts.

You can also gain these from the material shop in the same building, from buying steel cavalry chests in the items store (for gems) or from opening steel cavalry chests that you get from Clash of Provinces rewards. In these cases though the items you get are random and the tendons are the rare part so you get less of them.